The invention relates to a valve for controlling fluids.
EP-0 477 400 A1 describes a valve of this kind in which an actuating piston of the valve member is movably disposed in a stepped bore of the valve housing, in a small diameter part of the stepped bore. A larger piston that can be moved by means of a piezoelectric actuator is disposed in a larger diameter part of the stepped bore. A hydraulic chamber filled with a pressure medium is disposed between the two pistons so that a movement of the piezoelectric actuator is hydraulically transmitted. This means that when the larger piston is moved a certain distance by the piezoelectric actuator, the actuating piston of the valve member executes a stroke that is enlarged by the transmission ratio of the piston diameter because the piston of the piezoelectric actuator has a larger surface area than the actuating piston of the valve member. In this connection, the valve member, the actuating piston of the valve member, the piston moved by the piezoelectric actuator, and the piezoelectric actuator are disposed in series on a common axis.
In valves of this kind, there is the problem that a higher transmission ratio, e.g. approximately 1:8, can only be achieved with correspondingly high error tolerances since external influences such as temperature increase in the engine compartment or also losses in the pressure medium, as error components, have a very strong influence on the transmission ratio.
For a long while now, divided hydraulic pressure intensifiers have been used to prevent this problem when there is a high transmission ratio. In these divided hydraulic pressure intensifiers, two hydraulic pressure intensifiers are connected in series so that their transmission ratios can be added. As a result, each individual hydraulic pressure intensifier has lower demands placed on it so that even with external influences, the preset transmission ratio, approximately 1:8, is nevertheless maintained. On the other hand, these divided hydraulic pressure intensifiers have turned out to be susceptible to oscillation to which in turn leads to imprecision.
The valve according to the invention, has the advantage over the prior art that the combination of hydraulic and mechanical pressure intensifiers solves the oscillation problem in a simple form since the hydraulic pressure intensifier no longer has to be divided. At the same time, the hydraulic pressure intensifier performs the temperature compensation between the piezoelectric actuator and the housing so that the temperature change disadvantages which are common with purely mechanical pressure intensifiers can be compensated for. Consequently, the valve for controlling fluids according to the invention can be used to achieve a uniform reproducibility of injections so that precisely defined injection times and/or injection quantities of fuel can be assured. Furthermore, a mechanical pressure intensifier is simply designed and can be inexpensively produced so that advantages that pertain to it can also be achieved with regard to a second hydraulic pressure intensifier. Moreover, the mechanical pressure intensifier does not have any problems due to contamination of hydraulic fluid so that the maintenance costs with regard to a second hydraulic pressure intensifier are considerably reduced.
In a particularly advantageous embodiment, the mechanical pressure intensifier can have a lever which transmits the stroke of the piston element to the valve member. Levers can be simply designed and inexpensively produced and result in a rugged embodiment of the valve.
The lever is advantageously supported on a support which divides the lever into two lever arms. In a simple form, this presents the possibility of adapting the transmission ratio to the corresponding preset conditions. At the same time, the cooperation of the lever and support can achieve a precise transmission of the stroke of the piezoelectric actuator onto the valve member. In this connection, a lever arm length ratio of 4:1 has turned out to be particularly advantageous.
The piston element is preferably embodied as a push rod. In the field in question, push rods are standard components which can be simply and inexpensively produced.
In the valve for controlling fluids according to the invention, the transmission surface area of the push rod and the associated surface area of the piezoelectric actuator produce a transmission ratio of 2:1. This low transmission ratio of the hydraulic pressure intensifier assures a low oscillation susceptibility of the hydraulic pressure intensifier. On the other hand, this transmission ratio is added to the predetermined lever arm length ratio of 4:1, resulting in a total transmission ratio of 8:1 so that the desired transmission ratio is achieved without oscillation problems.
In addition, in the valve for controlling fluids according to the invention, a piston associated with the piezoelectric actuator can be disposed between the pressure chamber and the piezoelectric actuator. On the one hand, this piston transmits the longitudinal expansion from the piezoelectric actuator to the push rod and on the other hand, prevents the piezoelectric actuator from coming into contact with the hydraulic fluid. In order to further improve this seal between the piezoelectric actuator and hydraulic fluid, the transition region between the piezoelectric actuator and the associated piston in sealed with a sealing element.